1. Field of the Invention
The present invention relates to a controlling device for a tape-shaped recording medium and a tracking control method for a tape-shaped recording medium. More particularly, the present invention relates to a controlling device for controlling the relative speed of the transporting speed of a tape-shaped recording medium and the peripheral speed of a rotational drum.
2. Background of the Invention
Recording and/or recording apparatuses such as digital audio tape recorders and external storage devices for computers employing magnetic tape as a recording medium for storing audio data or computer data are well known. In such apparatus, a magnetic tape is wound helically or wrapped around the outer peripheral of the rotary drum to an angular extent of 90.degree.. In this condition, the magnetic tape is transported and the rotary drum is rotationally driven. As a result, high density data is recorded on the magnetic tape by a head of the rotary drum in a helical scanning method.
In this case, with the aid of the rotary drum, there are recorded on the magnetic tape T a plurality of tracks TK.sub.A,TK.sub.B inclined relative to the proceeding direction of the magnetic tape T, as schematically shown in FIG. 1. In FIG. 1, alternate tracks TK.sub.A,TK.sub.B are formed and recorded by magnetic heads A and B built in the rotary drum, respectively. These tracks TK.sub.A, TK.sub.B have opposite azimuths. Information signals ATF1, ATF2 for use in an automatically tracking control (hereinafter referred to as ATF signals) are recorded in predetermined positions as ATF (Automatic Track Finding) areas of each track. These ATF signals are used as a tracking control which is utilized in the tracking operation to control a capstan motor and thus control the transportation speed of the magnetic tape T. In the conventional digital audio tape recorder, the tracking control is executed according to detection of these ATF signals.
In the other tracking control, which is used in the external storage device, there is used a signal recorded in a predetermined position of the track which is a sync signal such as a block sync signal. In the external storage device, a plurality of block data is recorded in the tracks. It is the block sync signal that is used for the tracking control which is positioned at the lower end position of the tracks. Hereinafter, the signal recorded in the predetermined position of the track is referred to as a TATF signal. During reproduction, the magnetic heads must accurately scan the tracks of the magnetic tape T. In the tracking control using the TATF signals, the time or period is measured from the time the magnetic head on the starting rotary drum is at a reference position at the magnetic head arrives to the time the pattern of the TATF signal, and the measured time is compared with a reference value. An error component is derived from the compared result become to a servo error information. The transportation speed of the magnetic tape T is controlled by controlling the rotational speed of the capstan motor which is transporting the magnetic tape according to the servo error information. In other words, the relative velocity between the rotational speed of the rotary drum and the transportation speed of the magnetic tape is controlled by the adjustment of the transportation speed of the magnetic tape so as to obtain desirable tracking conditions.
For example, as shown in FIG. 2A, the position of the rotary drum is taken to be the reference position when the magnetic head HD is in the position shown in FIG. 2A with respect to the track TK on the magnetic tape T. A time T.sub.1 when the rotational drum is in the reference phase position may then be detected by, for example, adopting a construction where a pulse signal is generated from a pulse generator (PG) provided at a drum motor for rotationally driving the rotary drum, at the time when the rotating rotary drum is in the reference phase position. After this, the magnetic head HD comes into contact with the magnetic tape T, scans the track TK and detects the signal pattern for the TATF signal as the reproducing data, the timing of which is taken as the time T.sub.2. The time necessary from the time T.sub.1 to the time T.sub.2 is taken as T.sub.S, as shown in FIGS. 2A and 2B. The time necessary from when the rotary drum is in the reference phase position to when the signal pattern for the TATF signal is detected that is, the time for the magnetic head HD to obtain preferable tracking conditions with respect to the track TK so that the head is tracing the center of the track TK is measured and set-up beforehand as the reference value. Desirable tracking conditions are therefore obtained when the measured time T.sub.S coincides with the reference value. On the other hand, when the time T.sub.S and the reference value are compared and a difference exists, a tracking shift proportional to this difference is occurring. Servo control can then be executed so as to remove this difference by influencing the transportation speed of the magnetic tape. However, with the tape recording and/or reproducing apparatus described above, the position of the average height of the tracks formed on the magnetic tape changes for various reasons. In FIG. 1, conditions are shown where the position of the average height of the each of the tracks TK.sub.A and TK.sub.B on the magnetic tape T has changed by .DELTA.h in regions AR1 and AR2. This may occur for various reasons such as temperature differences during recording, differences in mechanical conditions in discontinuous recording or recording using different equipment. For example, when the portion for the region AR2 is recorded by a different recorder after the portion for the region AR1 of FIG. 1 has been recorded, the position of the average height of the tracks often changes as shown in FIG. 1 due to the difference in mechanical conditions between the recorders. In the case where the magnetic tape with tracks shown in FIG. 1 is reproduced with a servo operation using a tracking control method, these changes cause extremely undesirable results. Namely, the time T.sub.S, which is a time from when the rotational drum is in the reference phase position to when the signal pattern for the TATF signal is detected, changes significantly between the region AR1 and the region AR2. For example, when the reproduction is shifted to the region AR2 from the region AR1 for which desirable tracking control is being performed by comparing a certain reference value with the measured value T.sub.S, the measured value T.sub.S will thereafter include the time corresponding to the variation .DELTA.h in the height position as an error for the previous reference value. The tracking control is therefore then exerted toward out of control, by which the reproducing error rate is deteriorated and data correction and the reproduction become impossible. The above-mentioned tracking servo method is therefore extremely ineffective against variation in the position of the average track height. In the case of an apparatus for recording and/or reproducing computer data, when reproduction becomes impossible due to variation in the position of the average track height, the tape is rewound back to the point where the reproduction became impossible and a new reference value is measured, the tape is then rewound again and tracking control can be carried out using the new reference value. However, this considerably delays the reproducing operation. With devices that playback music data or moving image data, that is, an apparatus or devices that demand immediate reproduction, this kind of re-try operation cannot be carried out and the above kind of tracking control method may not be adopted.